U.S. patent number 5,717,709 [Application Number 08/668,086] was granted by the patent office on 1998-02-10 for semiconductor light-emitting device capable of having good stability in fundamental mode of oscillation, decreasing current leakage, and lowering oscillation threshold limit, and method of making the same.
This patent grant is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Kazuaki Sasaki, Osamu Yamamoto.
United States Patent |
5,717,709 |
Sasaki , et al. |
February 10, 1998 |
Semiconductor light-emitting device capable of having good
stability in fundamental mode of oscillation, decreasing current
leakage, and lowering oscillation threshold limit, and method of
making the same
Abstract
A semiconductor laser device includes a substrate having one of
p- and n-conductivity types, and a current constrictive layer
formed on a surface of the substrate and having the other type of
conductivity. The current constrictive layer has a through-channel
extending to the surface of the substrate for defining a current
path in a direction perpendicular to the surface of the substrate.
The through-channel is of a belt-like pattern extending in a
direction perpendicular to end surfaces of the substrate. A third
cladding layer having the one type of conductivity is filled in the
through-channel, a surface of the third cladding layer being flush
with a surface of a current constrictive layer. A first cladding
layer, an active layer, and a second cladding layer which
constitute a double heterostructure are formed over the third
cladding layer and current constrictive layer.
Inventors: |
Sasaki; Kazuaki (Yao,
JP), Yamamoto; Osamu (Nara, JP) |
Assignee: |
Sharp Kabushiki Kaisha (Osaka,
JP)
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Family
ID: |
26355179 |
Appl.
No.: |
08/668,086 |
Filed: |
June 19, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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253363 |
Jun 3, 1994 |
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Foreign Application Priority Data
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Jun 4, 1993 [JP] |
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5-134449 |
Feb 15, 1994 [JP] |
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6-018500 |
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Current U.S.
Class: |
372/46.01;
257/E33.067 |
Current CPC
Class: |
H01L
33/145 (20130101); H01S 5/2232 (20130101); H01S
5/18352 (20130101); H01S 5/2206 (20130101); H01S
5/3202 (20130101); H01S 5/32308 (20130101); H01S
5/4031 (20130101); H01S 2304/12 (20130101) |
Current International
Class: |
H01L
33/00 (20060101); H01S 5/00 (20060101); H01S
5/223 (20060101); H01S 5/323 (20060101); H01S
5/32 (20060101); H01S 5/40 (20060101); H01S
5/183 (20060101); H01S 5/22 (20060101); H01S
003/18 () |
Field of
Search: |
;372/46,48 ;257/98 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0342983 |
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Nov 1989 |
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EP |
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0351839 |
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Jan 1990 |
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EP |
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0395436 |
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Oct 1990 |
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EP |
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3827961 |
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Mar 1989 |
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DE |
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60-130882 |
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Jul 1985 |
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JP |
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60-0130882 |
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Jul 1985 |
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JP |
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61-281562 |
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Dec 1986 |
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JP |
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04-369882 |
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Dec 1992 |
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JP |
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4-369882 |
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Dec 1992 |
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JP |
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2247347 |
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Feb 1992 |
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GB |
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Other References
Dzurlo et al., "MOCVD growth of A1GaAs/GaAs structures on nonplanar
[111] substrates: Evidence for lateral gas phase diffusion" Journal
of Electronic Materials (1990) 19(12):1267-1372. (no month
available). .
Lee et al., "Buried-ridge striped planar GaA1As/GaAs lasers with of
wide range a effective index steps" Applied Physics Letters (1990)
56(7):599-601. (Feb. 12). .
Yamaguchi, K. et al., "Lateral growth on [111]B GaAs substrates by
metalorganic chemical vapor deposition" Journal of Crystal Growth
(1989) 94:203-207. (no month available)..
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Primary Examiner: Bovernick; Rodney B.
Assistant Examiner: Song; Yisun
Attorney, Agent or Firm: Morrison & Foerster
Parent Case Text
This application is a file wrapper continuation of application Ser.
No. 253,363, filed Jun. 3, 1994, now abandoned.
Claims
What is claimed is:
1. A semiconductor light-emitting device including a substrate
having one of p- and n-conductivity types, a current constrictive
layer formed on a surface of the substrate and having the other of
the p- and n-conductivity types, the current constrictive layer
having at least one through-channel extending to the surface of the
substrate for defining a current path in a direction perpendicular
to the surface of the substrate, and a double heterostructure
formed on the current constrictive layer and including a first
cladding layer, an active layer and a second cladding layer,
characterized in that:
the through-channel is of a belt-like pattern which extends
perpendicularly to end surfaces of the substrate; and the
semiconductor light-emitting device comprises
a third cladding layer having the one type of conductivity, at
least one portion of the third cladding layer being filled in the
through-channel, and the at least one portion of the third cladding
layer having an upper surface flush with an upper surface of the
current constrictive layer.
2. A semiconductor light-emitting device as set forth in claim 1,
wherein the third cladding layer has an extended portion covering
the surface of the current constrictive layer, the extended portion
having a thickness set thinner than that of the portion of the
third cladding layer filled in the through-channel.
3. A semiconductor light-emitting device as set forth in claim 1,
further comprising an extension to the current constrictive layer
having the other type of conductivity and filled in a peripheral
portion of the through-channel, the extension having a surface
flush with the surface of the current constrictive layer; and
the third cladding layer being disposed inward of the extension to
the current constrictive layer within the through-channel.
4. A semiconductor light-emitting device as set forth in claim 1,
wherein a plurality of the through-channels are formed in the
current constrictive layer, the third cladding layer being embedded
in each of the through-channels.
5. A semiconductor light-emitting device as set forth in claim 1,
further comprising at least one non-through channel formed in
parallel with the through-channel in the current constrictive layer
to a depth not greater than the depth of the current constrictive
layer; and
a fourth cladding layer having the one type of conductivity and
filled in the non-through channel, the fourth cladding layer having
a surface flush with the surface of the current constrictive
layer.
6. A surface output type semiconductor including a substrate having
one of p- and n-conductivity types, a current constrictive layer
formed on a surface of the substrate and having the other of the p-
and n-conductivity types, the current constrictive layer having a
through-channel extending to the surface of the substrate for
defining a current path in a direction perpendicular to the surface
of the substrate, and a double heterostructure formed on the
current constrictive layer and including a first cladding layer, an
active layer and a second cladding layer, characterized in
that:
the through-channel is of a circular pattern; and the semiconductor
light-emitting device comprises
a third cladding layer having the one type of conductivity and
filled in the through-channel, the third cladding layer having an
upper surface flush with an upper surface of the current
constrictive layer.
7. A surface output type semiconductor as set forth in claim 6,
wherein the layer forming the double heterostructure is configured
to be frusto-conical.
8. A semiconductor light-emitting device as set forth in claim 1,
wherein the substrate is a GaAs substrate, the surface of the
substrate being (111)B face or a face offset to the (111)B face
which is a main face;
the current constrictive layer is comprised of GaAs or AlGaAs;
and
the third cladding layer is comprised of AlGaAs.
9. A surface output type semiconductor as set forth in claim 6,
wherein the substrate is a GaAs substrate, the surface of the
substrate being (111)B face or a face offset to the (111)B face
which is a main face;
the current constrictive layer is comprised of GaAs or AlGaAs;
and
the third cladding layer is comprised of AlGaAs.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor light-emitting
devices and methods for making such devices. More particularly, the
invention relates to a short wavelength semiconductor laser which
emits light beams of red to orange colors and a light emitting
diode (LED) having a wavelength band of red to green colors, and
methods for making the same.
2. Description of the Prior Art
Recently, red semiconductor lasers having a wavelength band of 630
to 680 nm constructed of AlGaInP have been receiving attention as a
promising source of light for POS, as well as for high definition,
high density photomagnetic disks. Indeed, researches and
developments have been made in this connection. When such laser is
used for disks, fundamental mode stability and good optical
characteristics such as astigmatism and so on, in particular are
important. For this reason, there exists a need for a semiconductor
laser of the refractive index guide type which confines light beams
within the region of oscillation.
Refractive index guide type semiconductor laser devices of 680 nm
wavelength band have hitherto been known including one shown in
FIG. 11 which is of the effective refractive index guide type, and
another shown in FIG. 12 which is of the real refractive index
guide type. FIG. 11 is a sectional view showing the semiconductor
laser device of the effective refractive index guide type, and FIG.
12 is a sectional view showing the semiconductor laser device of
the real refractive index guide type. The semiconductor laser
device shown in FIG. 11 is fabricated in such a way that on an
n-GaAs substrate 131 having (100) face as a main face are grown an
n-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P cladding layer (1.5
.mu.m thick) 133, a non-doped Ga.sub.0.5 In.sub.0.5 P active layer
(0.05 .mu.m thick) 134, a p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5
In.sub.0.5 P cladding layer (1.5 .mu.m thick) 135, and a
p-Ga.sub.0.5 In.sub.0.5 P intermediate layer 136 according to a
MOCVD (Metal Organic Chemical Vapor Deposition) process. Then, the
intermediate layer 136 and an upper portion of the cladding layer
135 are removed by etching, leaving a centrally located ridge
portion 141. Subsequently, n-GaAs current constrictive layers 132
are grown on both sides of the ridge portion 141 and, in addition,
a p-GaAs contact layer 137 (2 .mu.m thick) is grown over the entire
region. Finally, electrodes 139, 140 are formed respectively on the
underside of the substrate 131 and on the surface of the contact
layer 137. In such a semiconductor laser, the current constrictive
layers 132 limit current passage to decrease ineffective current
and cause a substantially large mode loss relative to a higher
order mode of oscillation. Thus, an oscillation mode of higher
order is suppressed so that oscillation of the fundamental mode is
steadily maintained in oscillation region 134a to a high light
output.
The semiconductor laser device shown in FIG. 12 is fabricated in
such a way that on a p-GaAs substrate 101 having (100) face is
formed an n-GaAs current constrictive layer 102 in which a channel
102b is formed reaching from the surface of the layer 102 into the
substrate 101. Then, a p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5
P cladding layer (1.8 .mu.m thick) 103, a non-doped Ga.sub.0.5
In.sub.0.5 P active layer (0.05 .mu.m) 104, an n-(Al.sub.0.5
Ga.sub.0.5).sub.0.5 In.sub.0.5 P cladding layer (1.5 .mu.m thick)
105, and an n-Ga.sub.0.5 In.sub.0.5 P contact layer (1 .mu.m thick)
106 are sequentially formed over the layer 102 according to the
MOCVD process (where each layer thickness value denotes the
thickness of the respective layer in the channel). Finally,
electrodes 109, 110 are formed respectively on the underside of the
substrate 101 and on the surface of the contact layer 106. In the
stage of growth according to the MOCVD process, the layer being
grown usually reflects the configuration of the base layer.
Therefore, the active layer 104 is of such a configuration that it
is largely bent above the edge of the channel 102b, that is, above
corresponding ends of the current constrictive layer 102, whereby a
real refractive index guide structure is formed. According to this
arrangement, possible loss in the fundamental mode is reduced,
which results in reduced threshold oscillation value and increased
differential efficiency.
Unfortunately, however, the semiconductor layer shown in FIG. 11
involves a problem that the stability of the fundamental mode
depends largely on the thickness (residual thickness) d of the
cladding layer portions remaining at both sides of the ridge 141.
This means that when etching variations are so wide that the
residual thickness d substantially exceeds 0.3 .mu.m, the
fundamental mode is rendered unstable. (It is noted in this
connection that an optimum value of residual thickness d is
approximately 0.2 .mu.m.) The same is true with the case in which
residual thickness d differs on opposite sides of the ridge 141.
Another problem is that in the stage of current constrictive layer
132 growing, the base layer for such growth is a layer including
Al, that is, the p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P
layer 135, which fact is likely to be a cause of oxidation so that
the quality of a regrown interface 135a will be unfavorably
affected. This results in current leaks which in turn lead to an
increase in the oscillation threshold value. Typically, a
non-coated device having a resonator length of 400 .mu.m has an
oscillation threshold value of 45 mA and a kink level of about 25
mW.
The semiconductor laser device shown in FIG. 12, wherein the active
layer 104 is largely bent at ends of the current constrictive layer
102 to provide a real refractive index guide construction, has
smaller losses in both fundamental and higher order modes. This
presents a problem that the kink level becomes rather lowered.
Typically, a non-coated device having a resonator length of 400
.mu.m has an oscillation threshold value of 25 to 30 mA and a kink
level of about 20 mW.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to provide a
semiconductor light-emitting device having a current constrictive
layer which has improved performance characteristics, and a method
of making the same. Semiconductor light-emitting devices to which
the invention is directed include, in particular, semiconductor
laser devices and light-emitting diodes.
In order to achieve the aforementioned object, there is provided a
semiconductor light-emitting device including a substrate having
one of p- and n-conductivity types, a current constrictive layer
formed on a surface of the substrate and having the other of the p-
and n-conductivity types, the current constrictive layer having at
least one through-channel extending to the surface of the substrate
for defining a current path in a direction perpendicular to the
surface of the substrate, and a double heterostructure formed on
the current constrictive layer and including a first cladding
layer, an active layer and a second cladding layer, characterized
in that:
the through-channel is of a belt-like pattern which extends
perpendicularly to end surfaces of the substrate; and the
semiconductor light-emitting device comprises
a third cladding layer having the one type of conductivity, at
least one portion of the third cladding layer being filled in the
through-channel, and the at least one portion of the third cladding
layer having a surface flush with a surface of the current
constrictive layer.
The semiconductor light emitting device includes the current
constrictive layer formed on the surface of the substrate having
one of the p- and n-conductivity types, the current constrictive
layer having the other of the p- and n-conductivity types. The
current constrictive layer has the through-channel of a belt-like
pattern extending perpendicularly to the end surfaces of the
substrate. Therefore, this semiconductor light-emitting device can
constitute a refractive index guide type semiconductor laser
device. The semiconductor laser device has the third cladding layer
of the one conductivity type filled in the through-channel of the
current constrictive layer, the surface of the third cladding layer
being flush with the surface of the current constrictive layer. A
first cladding layer, an active layer and a second cladding layer
which constitute a double heterostructure are formed by a known
growth method, for example, MOCVD process, flatly over the current
constrictive and third cladding layers in a well-controlled manner,
without involving any stage of etching. Therefore, the first
cladding layer, active layer and second cladding layer involve
almost no variation in thickness. This insures good stability in
the fundamental mode of laser oscillation. The fact that no etching
stage is involved after the formation of the third cladding layer
eliminates the possibility of any growth interface being oxidized.
This results in decreased current leakage and lowered oscillation
threshold limit. The first cladding layer is usually designed to be
relatively thin and in no case does this result in decreased
higher-order mode loss. Therefore, good improvement is achieved in
kink level. In this way, the semiconductor laser device having
improved performance characteristics can be obtained.
According to an embodiment, the third cladding layer has an
extended portion covering the surface of the current constrictive
layer, the extended portion having a thickness set thinner than
that of the portion of the third cladding layer filled in the
through-channel.
According to the above arrangement, the third cladding layer has
the extended portion covering the surface of the current
constrictive layer, and the extended portion has a thickness set
thinner than that of the portion filled in the through-channel. In
this case, when the layers which make a double heterostructure are
formed using, for example, the MOCVD process, the active layer
becomes slightly bent adjacent edge of the through-channel. As a
result, a waveguide arrangement is obtained which has both a
characteristic of an effective refractive index guide structure
utilizing the light absorption of the substrate (current
constrictive layer) and a characteristic of a real refractive index
guide structure utilizing a bend of the active layer, that is, the
advantages of both of the prior art arrangements described. In
other words, the semiconductor laser device can reduce mode loss
relative to the fundamental mode and increase mode loss relative to
the higher-order mode, so that the fundamental mode is further
stabilized.
A semiconductor light-emitting device of an embodiment comprises
extension to the current constrictive layer having the other type
of conductivity and filled in a peripheral portion of the
through-channel, the extension having a surface flush with the
surface of the current constrictive layer; and
the third cladding layer being disposed inward of the extension to
the current constrictive layer within the through-channel.
With the above arrangement, the extension to the current
constrictive layer which has the other type of conductivity is
filled in the peripheral portion of the through-channel, and the
surface of the extension is flush with the surface of the current
constrictive layer. Further, the third cladding layer is filled
inward of the extension to the current constrictive layer within
the through-channel. Therefore, in operation a width of current
injection is proportionally reduced by the extension to the current
constrictive layer. Therefore, an oscillation threshold value
thereof is further reduced and, in addition, some astigmatism
reduction is achieved.
According to an embodiment, a plurality of the through-channels are
formed in the current constrictive layer, the third cladding layer
being embedded in each of the through-channels.
With the above arrangement, the plurality of through-channels are
formed in the current constrictive layer, with the third cladding
layer being embedded in each of the through-channels. Thus, in
operation a plurality of oscillation regions develop according to
the number of current paths formed by the through-channels. This
provides a semiconductor laser array.
A semiconductor light-emitting device of an embodiment comprises at
least one non-through channel formed in parallel with the
through-channel in the current constrictive layer to a depth not
greater than the depth of the current constrictive layer; and
a fourth cladding layer having the one type of conductivity and
filled in the non-through channel, the fourth cladding layer having
a surface flush with the surface of the current constrictive
layer.
According to the arrangement, the at least one non-through channel
is formed in parallel with the through-channel in the current
constrictive layer to a depth not greater than the depth of the
current constrictive layer, and further a fourth cladding layer
having the one type of conductivity is filled in the non-through
channel, the surface of the fourth cladding layer being flush with
the surface of the current constrictive layer. In this case, any
strain that is applied to the active layer because of the
respective layers stacked on the substrate is dispersed over the
non-through channel. Thus, the strain exerted on the oscillation
region over the through-channel is alleviated so that a long-term
reliability of the device can be enhanced.
Also, there is provided a semiconductor light-emitting device
including a substrate having one of p- and n-conductivity types, a
current constrictive layer formed on a surface of the substrate and
having the other of the p- and n-conductivity types, the current
constrictive layer having a through-channel extending to the
surface of the substrate for defining a current path in a direction
perpendicular to the surface of the substrate, and a double
heterostructure formed on the current constrictive layer and
including a first cladding layer, an active layer and a second
cladding layer, characterized in that:
the through-channel is of a circular pattern; and the semiconductor
light-emitting device comprises
a third cladding layer having the one type of conductivity and
filled in the through-channel, the third cladding layer having a
surface flush with the surface of the current constrictive
layer.
The semiconductor light-emitting device includes the substrate
having one of the p- and n-conductivity types, and the current
constrictive layer formed on the surface of the substrate and
having the other of the p- and n-conductivity types, the current
constrictive layer having the through-channel of a circular pattern
formed therein. Accordingly, this semiconductor light-emitting
device can constitute a surface output type light-emitting diode.
The light-emitting diode comprises the third cladding layer having
the one type of conductivity and filled in the through-channel, the
third cladding layer having a surface flush with the surface of the
current constrictive layer. Therefore, the first cladding layer,
active layer and second cladding layer which constitute the double
heterostructure are formed by a known growth method, for example,
MOCVD process, flatly over the current constrictive and third
cladding layers in a well-controlled manner, without involving any
stage of etching. Therefore, the first cladding layer, active layer
and second cladding layer involve almost no variation in thickness.
This insures good stability in radiation intensity-applied current
characteristics. The fact that no etching stage is involved after
the formation of the third cladding layer eliminates the
possibility of any growth interface being oxidized. This results in
decreased current leakage and increased light emission intensity.
In this way, the light-emitting diode having improved performance
characteristics can be obtained.
Where the layer forming the double heterostructure is configured to
be frusto-conical, the efficiency of light output of the device can
be enhanced.
According to an embodiment, the substrate is a GaAs substrate, the
surface of the substrate being (111)B face or a face offset to the
(111)B face which is a main face;
the current constrictive layer is comprised of GaAs or AlGaAs;
and
the third cladding layer is comprised of AlGaAs.
According to the arrangement, the substrate is the GaAs substrate
having the (111)B face or the face offset to the (111)B face which
is a main face; the current constrictive layer is comprised of GaAs
or AlGaAs; and the third cladding layer is comprised of AlGaAs. In
this case, as will be described hereinafter, it is possible to grow
the third cladding layer comprised of AlGaAs having the one type of
conductivity within the through-channel while the substrate is kept
at a temperature of not more than 720.degree. C. in such a manner
that the surface of the third cladding layer becomes flush with the
surface of the current constrictive layer thereby to fill the
through-channel. Therefore, the first cladding layer, active layer,
and second cladding layer which constitute a double heterostructure
can be grown by the known growth technique flatly over the
substrate in a well controlled manner.
There is provided a method of making a semiconductor light-emitting
device comprising the steps of:
forming on a surface of a GaAs substrate having one of p- and
n-conductivity types a current constrictive layer comprised of GaAs
or AlGaAs and having the other of the p- and n-conductivity types,
the surface being (111)B face or a face offset to the (111)B face
which is a main face;
forming in the current constrictive layer a through-channel of a
predetermined pattern which extends from a surface of the current
constrictive layer to the substrate;
growing a third cladding layer comprised of AlGaAs and having the
one type of conductivity within the through-channel while the
substrate is kept at a temperature of not more than 720.degree. C.
to fill the through-channel with the third cladding layer in such a
manner that the surface of the third cladding layer becomes flush
with the surface of the current constrictive layer; and
successively growing a first cladding layer, an active layer, and a
second cladding layer over the substrate to form a double
heterostructure.
According to the method of making a semiconductor light-emitting
device, on the surface of the GaAs substrate having the one of the
p- and n-conductivity types is formed the current constrictive
layer comprised of GaAs or AlGaAs and having the other of the p-
and n-conductivity types, the substrate surface having (111)B face
or a face offset to the (111)B face which is a main face. The
through-channel of the predetermined pattern which extends from the
surface of the current constrictive layer to the substrate is then
formed in the current constrictive layer. Then, the third cladding
layer comprised of AlGaAs and having the one type of conductivity
is grown within the through-channel while the substrate is kept at
the temperature of not more than 720.degree. C. This enables the
third cladding layer to be grown so that its surface becomes flush
with the surface of the current constrictive layer thereby to fill
the through-channel. Therefore, the first cladding layer, active
layer, and second cladding layer can be formed flatly over the
third cladding layer and current constrictive layer in a well
controlled manner to form a double heterostructure. Thus, it is now
possible to fabricate semiconductor light-emitting devices, such as
a semiconductor laser device and a light-emitting diode, which have
good characteristic improvement over the prior art devices.
Also, there is provided a method of making a semiconductor
light-emitting device comprising the steps of:
forming on a surface of a GaAs substrate having one of p- and
n-conductivity types a current constrictive layer comprised of GaAs
or AlGaAs and having the other of the p- and n-conductivity types,
the surface being (111)B face or a face offset to the (111)B face
which is a main face;
forming in the current constrictive layer a through-channel of a
predetermined pattern which extends from a surface of the current
constrictive layer to the substrate;
growing a third cladding layer comprised of AlGaAs and having the
one type of conductivity while the substrate is kept within a
temperature range of 720.degree. C. to 740.degree. C. in such a
manner that one portion of the third cladding layer which fills the
through-channel has a surface flush with the surface of the current
constrictive layer and that the third cladding layer has an
extended portion overlying the surface of the current constrictive
layer and being thinner than the fill portion; and
successively growing a first cladding layer, an active layer, and a
second cladding layer over the substrate to form a double
heterostructure.
According to the method of making a semiconductor light-emitting
device, after the through-channel is formed in the current
constrictive layer, the third cladding layer comprised of AlGaAs
and having the one type of conductivity is grown while the
substrate is kept within the temperature range of 720.degree. C. to
740.degree. C. so as to fill the through-channel in such a manner
that the surface of that portion of the third cladding layer which
fills the through-channel is flush with the surface of the current
constrictive layer and that the third cladding layer has an
extended portion overlying the surface of the current constrictive
layer which is thinner than the fill portion. Therefore, when the
layers constituting the double heterostructure are formed on the
current constrictive layer using, for example, the MOCVD technique,
the active layer is configured to be slightly bent adjacent the
edge of the through-channel. As a result, the semiconductor
light-emitting device made has a waveguide arrangement featuring
both the characteristic of an effective refractive index guide
structure utilizing the light absorption of the substrate (current
constrictive layer) and the characteristic of a real refractive
index guide structure utilizing the bend of the active layer, that
is, the advantages of both of the prior art arrangements shown. In
other words, the semiconductor laser device can reduce mode loss
relative to the fundamental mode and increase mode loss relative to
the higher-order mode, so that the fundamental mode is further
stabilized. Furthermore, possible current leaks are reduced and the
oscillation threshold limit is lowered.
Also, there is provided a method of making a semiconductor
light-emitting device comprising the steps of:
forming on a surface of a GaAs substrate having one of p- and
n-conductivity types a current constrictive layer comprised of GaAs
or AlGaAs and having the other of the p- and n-conductivity types,
the surface being (111)B face or a face offset to the (111)B face
which is a main face;
forming in the current constrictive layer a through-channel of a
predetermined pattern which extends from a surface of the current
constrictive layer to the substrate;
growing in a peripheral portion of the through-channel an extension
to the current constrictive layer which is comprised of GaAs or
AlGaAs and has the other type of conductivity while the substrate
is kept at a temperature of not more than 720.degree. C., in such a
manner that a surface of the extension is flush with the surface of
the current constrictive layer thereby to reduce a width of the
through-channel;
growing a third cladding layer comprised of AlGaAs and having the
one type of conductivity within the through-channel and internally
of the extension to the current constrictive layer while the
substrate is kept at a temperature of not more than 720.degree. C.,
to fill the third cladding layer inside the extension in such a
manner that a surface of the third cladding layer is flush with the
surface of the current constrictive layer; and
successively growing a first cladding layer, an active layer, and a
second cladding layer over the substrate to form a double
heterostructure.
According to the method of making a semiconductor light-emitting
device, the extension to the current constrictive layer which has
the other type of conductivity and whose surface is flush with the
surface of the current constrictive layer is filled in the
peripheral portion of the through-channel in the current
constrictive layer, and the third cladding layer is embedded
internally of the extension to the current constrictive layer.
Therefore, when the semiconductor light-emitting device thus made
is in operation, the width of current injection is proportionally
reduced by the extension to the current constrictive layer. With
such semiconductor light-emitting device, and semiconductor laser
device in particular, therefore, the oscillation threshold value is
further reduced and, in addition, some astigmatism reduction is
achieved.
Furthermore, there is provided a method of making a semiconductor
light-emitting device comprising the steps of:
forming on a surface of a GaAs substrate having one of p- and
n-conductivity types a current constrictive layer comprised of GaAs
or AlGaAs and having the other of the p- and n-conductivity types,
the surface being (111)B face or a face offset to the (111)B face
which is a main face;
forming in the current constrictive layer a plurality of
through-channels of a predetermined pattern which extend from a
surface of the current constrictive layer to the substrate;
growing third cladding layers comprised of AlGaAs and having the
one type of conductivity within the respective through-channels
while the substrate is kept at a temperature of not more than
720.degree. C., in such a manner that a surface of each of the
third cladding layers is flush with the surface of the current
constrictive layer, thereby filling the through-channels; and
successively growing a first cladding layer, an active layer, and a
second cladding layer over the substrate to form a double
heterostructure.
According to the method of making a semiconductor light-emitting
device, the through-channels are formed in plurality in the current
constrictive layer, and the third cladding layers are filled in
respective through-channels. With such a semiconductor
light-emitting device, in operation a plurality of oscillation
regions develop according to the number of current paths formed by
the through-channels. This provides a semiconductor laser
array.
Furthermore, there is provided a method of making a semiconductor
light-emitting device comprising the steps of:
forming on a surface of a GaAs substrate having one of p- and
n-conductivity types a current constrictive layer comprised of GaAs
or AlGaAs and having the other of the p- and n-conductivity types,
the surface being (111)B face or a face offset to the (111)B face
which is a main face;
forming in the current constrictive layer a non-through channel of
a predetermined pattern which is held within the current
constrictive layer;
forming in the current constrictive layer a through-channel of a
predetermined pattern which extends from a surface of the current
constrictive layer to the substrate;
growing third and fourth cladding layers comprised of AlGaAs and
having the one type of conductivity respectively within the
through-channel and non-through channel while the substrate is kept
at a temperature of not more than 720.degree. C., to respectively
fill the through-channel and non-through channel with the third and
fourth cladding layers in such a manner that the surfaces of the
third and fourth cladding layers are respectively flush with the
surface of the current constrictive layer; and
successively growing a first cladding layer, an active layer, and a
second cladding layer over the substrate to form a double
heterostructure.
According to the method of making a semiconductor light-emitting
device, the non-through channel having a depth of not more than the
depth of the current constrictive layer is provided in parallel
with the through-hole in the current constrictive layer, and the
fourth cladding layer having the one type of conductivity is
embedded in the non-through channel, with the surface of the fourth
cladding layer being made flush with the surface of the current
constrictive layer. By virtue of this arrangement, any strain
exerted on the oscillation region over the through-channel is
alleviated so that good improvement can be obtained in a long-term
reliability of the device.
Moreover, there is provided a method of making a semiconductor
light-emitting device comprising the steps of:
forming on a surface of a GaAs substrate having one of p- and
n-conductivity types a current constrictive layer comprised of GaAs
or AlGaAs and having the other of the p- and n-conductivity types,
the surface having (111)B face or a face offset to the (111)B face
which is a main face;
forming in the current constrictive layer a through-channel of a
circular pattern which extends from a surface of the current
constrictive layer to the substrate;
growing a third cladding layer comprised of AlGaAs and having the
one type of conductivity within the through-channel while the
substrate is kept at a temperature of not more than 720.degree. C.,
to fill the through-channel with the third cladding layer in such a
manner that a surface of the third cladding layer is flush with the
surface of the current constrictive layer;
successively growing a first cladding layer, an active layer, and a
second cladding layer over the substrate to form a double
heterostructure; and
working the layer forming the double heterostructure to a
frusto-conical configuration.
According to the method of making a semiconductor light-emitting
device, after the through-channel of a circular pattern which
extends from the surface of the current constrictive layer to the
substrate is formed in the current constrictive layer, the third
cladding layer having the one type of conductivity is grown within
the through-channel while the substrate is kept at a temperature of
not more than 720.degree. C., in such a manner that the surface of
the third cladding layer is flush with the surface of the current
constrictive layer, thereby filling the through-channel. Further,
the first cladding layer, active layer, and second cladding layer
are grown over the current constrictive layer and third cladding
layer to form the double heterostructure. Thus, a surface output
type light-emitting diode is constructed. In this case, the layers
which constitute the double heterostructure are flatly formed by a
known growth method, for example, MOCVD process, in a
well-controlled manner, without involving any stage of etching.
Therefore, almost no variation is involved in thickness. This
insures good stability in radiation intensity-applied current
characteristics. Furthermore, since no etching process is involved
after the formation of the third cladding layer, there is no
possibility of any growth interface being oxidized. This results in
decreased current leakage and increased light emission intensity.
In this way, a light-emitting diode having improved performance
characteristics can be obtained. Moreover, because of the fact that
the layers forming the double heterostructure are configured to be
frusto-conical, the efficiency of light output of the device can be
enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the
detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus are
not limitative of the present invention, and wherein:
FIG. 1 is a sectional view showing a semiconductor laser device
representing a first embodiment of the present invention;
FIG. 2 is a sectional view of a semiconductor laser device
representing a second embodiment of the invention;
FIG. 3 is a sectional view of a semiconductor laser device
representing a third embodiment of the invention;
FIG. 4 is a diagram showing the relations between growth rates of
AlGaAs, GaInP, and AlGaInP layers on (111) B surface of a GaAs
substrate and substrate temperatures;
FIGS. 5A, 5B, 5C, 5D and 5E are diagrammatic views explanatory of
the process for manufacturing the semiconductor laser device of the
first embodiment;
FIG. 6 is a sectional view showing a semiconductor laser device
representing a fourth embodiment of the invention;
FIG. 7 is a sectional view of a semiconductor laser device
representing a fifth embodiment of the invention;
FIG. 8 is a sectional view of a semiconductor laser device
representing a sixth embodiment of the invention;
FIG. 9 is a sectional view of a semiconductor laser device
representing a seventh embodiment of the invention;
FIG. 10A is a view showing in section a light emitting diode
according to an eighth embodiment of the invention;
FIG. 10B is a view showing in top plan the light emitting
diode;
FIG. 11 is a sectional view showing a conventional semiconductor
laser device of the effective refractive index guide type; and
FIG. 12 is a sectional view showing a conventional real refractive
index guide type semiconductor laser device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the invention will now be described in
detail with reference to the accompanying drawings. It should be
noted that hatchings for some parts are omitted for the sake of
simplicity in FIGS. 1-3, 5A-5E, 6-9, 10A, 11 and 12.
(First Embodiment)
FIG. 1 shows a section of a semiconductor laser device representing
a first embodiment of the present invention. The semiconductor
laser device includes a p-GaAs substrate 1 and, on (111)B face of
the substrate 1, an n-GaAs current constrictive layer (1 .mu.m
thick) 2, a p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P first
cladding layer (0.2 .mu.m thick) 3, a non-doped Ga.sub.0.5
In.sub.0.5 P active layer (0.05 .mu.m thick) 4, an n-(Al.sub.0.5
Ga.sub.0.5).sub.0.5 In.sub.0.5 P second cladding layer (1.5 .mu.m
thick) 5, and an n-Ga.sub.0.5 In.sub.0.5 P contact layer (0.5 .mu.m
thick) 6. Shown by 4a is an oscillation region (indicated by
oblique lines), and shown by 9 and 10 are electrodes. A belt-like
through-channel 2b extending perpendicularly to the section is
formed centrally in the current constrictive layer 2, and a
p-Al.sub.0.7 Ga.sub.0.3 As third cladding layer (1.3 .mu.m thick) 8
is embedded in the through-channel 2b. The surface 8a of the third
cladding layer 8 is flush with the surface 2a of the current
constrictive layer 2.
Before the steps of fabricating the device are discussed, basic
phenomenal factors will be explained. The present inventors have
found that when an Al.sub.0.7 Ga.sub.0.3 As layer, a Ga.sub.0.5
In.sub.0.5 P layer, an (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P
layer are grown by the MOCVD process on a GaAs substrate (of either
n or p conductivity type) having (111)B face as a main face, the
relationship between growth rate of each layer and substrate
temperature is as shown in FIG. 4. Materials used for layer growth
were TMG (trimethyl gallium), TMA (trimethyl aluminum), TMI
(trimethyl indium), AsH.sub.3 (arsine), and PH.sub.3 (phosphine).
In this experiment, for AlGaAs growth, V group element to III group
element supply ratio (hereinafter called "V/III ratio") was set at
80, and III group element supply quantity (the sum of TMG and TMA
supply quantities) was set at 1.6.times.10.sup.-5 mol/min. For
GalnP and AlGalnP growth, V/III ratio was set at 200, and III group
element supply quantity (the sum of TMG, TMA and TMI supply
quantities) was set at 2.0.times.10.sup.-5 mol/min. Similar
experiments were carried out with cases of Al.sub.x Ga.sub.1-x As
and (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P in which Al
proportions x were varied (x=0-1). Where III group element supply
quantity was same, almost same results as shown in FIG. 4 were
obtained. As is apparent from FIG. 4, when the substrate
temperature is lower than 720.degree. C., there is no growth of
AlGaAs layer on (111)B face of a GaAs substrate. GaInP and AlGaInP
layers can grow over a wide temperature range (650.degree. to
750.degree. C.), and their growth rates are almost constant, being
not dependent on the substrate temperature. The process for
manufacturing the device of the invention utilizes this
phenomenon.
The relationship between growth rate on (111)B face of a GaAs
substrate and substrate temperature has been known with respect to
GaAs growth (Journal of Crystal Growth, Vol. 94 (1989) p.p. 203-207
(hereinafter called "Reference 1"). FIG. 1 of the Reference 1 shows
that the growth rate of GaAs, as is the case with the growth rate
shown for Al.sub.x Ga.sub.1-x As in FIG. 4 of the present
application, is zero when the substrate temperature is lower than
720.degree. C. and crystal growth begins at a substrate temperature
of more than 720.degree. C. However, the description given in the
Reference 1 concerns only the characteristics of GaAs growth on
(111)B face of GaAs substrate, and does not relate to the growth of
AlGaAs, GaInP, and AlGaInP on (111)B face of GaAs substrate. As
will be described in detail hereinafter, the advantageous effect of
the present invention can only be achieved through the use of
AlGaAs and not GaAs as material for third cladding layer 8 shown in
FIG. 1. In other words, the present invention cannot be derived
from Reference 1, and the invention has been developed only through
the discovery of the phenomenal fact on the growth of AlGaAs,
GaInP, and AlGaInP as shown in FIG. 4.
For the purpose of making the semiconductor laser device, as FIG.
5A shows, an n-GaAs current constrictive layer 2 is grown 1 .mu.m
thick on (111)B face of p-GaAs substrate 1 by using liquid phase
growing technique. Then, as FIG. 5B shows, etching is carried out
to form through-channels 2b, 2b, . . . of 4 .mu.m wide and 1.3
.mu.m deep which extend from the surface 2a of the current
constrictive layer 2 to the p-GaAs substrate. In the present
example, the orientation of each through-channel 2b was
[100] direction.
Then, as FIGS. 5C and 5D show, a p-Al.sub.0.7 Ga.sub.0.3 As
cladding layer 8 was grown over the substrate 1 by the MOCVD
process. Growing conditions were: substrate temperature,
700.degree. C.; and V/III ratio, 80. As earlier explained with
reference to FIG. 4, under these growth conditions the rate of
growth on (111)B face of GaAs substrate 1 is almost zero.
Therefore, AlGaAs layer does not grow at the bottom of
through-channel 2b nor does it grow on the surface of the n-GaAs
current constrictive layer 2. Instead, as FIG. 5C shows, the AlGaAs
layers 8 grow inwardly from the side walls of through-channel 2b
and, as FIG. 5D shows, the through-channel is entirely filled when
fronts of the AlGaAs layers growing from the upper edges of the
side wall meet together. As a result, the surface 8a of each
cladding layer 8 becomes flush with the surface 2a of the current
constrictive layer 2, so that the surface side of the substrate 1
becomes flat. Subsequently, as FIG. 5E shows, a p-AlGaInP cladding
layer 3, a non-doped GaInP active layer 4, an n-AlGaInP cladding
layer 5, and an n-GaInP contact layer 6 were grown on the surface
side of the substrate by the MOCVD process. Growth conditions were:
substrate temperature, 700.degree. C.; and V/III ratio, 200. In
this case, the AlGaInP layers 3, 5 and GaInp layers 4, 6 exhibit a
growth pattern different from that seen with the AlGaAs layer, that
is, growth occurs on the (111)B face as well (growth rate is 1.7
.mu.m/hour under the aforesaid growth conditions). Then, electrodes
9, 10 were formed respectively on the underside of the substrate 1
and on the surface of the contact layer 6. Finally, the product was
split along each chain line in FIG. 5E into chips to give the same
semiconductor laser device as shown in FIG. 1.
According to the above described method, p-AlGaInP cladding layer 3
is grown by the MOCVD process in a well controlled manner and is
not subjected to etching. Therefore, little or no variation occurs
with respect to the thickness d of cladding layer 3. This leads to
good stability in the fundamental mode. The fact that no etching
step is involved eliminates the possibility of oxidation with any
growth interface. Current leakage is decreased, and oscillation
threshold limit is lowered. The cladding layer 3 on the current
constrictive layer 2 is so thin that higher-order mode loss can be
moderately maintained. This results in good improvement in the kink
level. True, semiconductor laser devices thus fabricated, in
noncoat condition and with resonator length of 400 .mu.m, had an
oscillation threshold limit of 40 mA and oscillated up to 50 mW
without kink. Its oscillation wavelength was 679 nm during 50 mW
output. As compared with the prior art semiconductor laser device
shown in FIG. 11 which, in the condition of noncoat and resonator
length 400 .mu.m, had an oscillation threshold value of 45 mA and a
kink level of 25 mW, this is considerable improvement or two-fold
improvement in kink level.
For purposes of comparison, a device having a sectional
construction identical with the device shown in FIG. 1 was made
using GaAs as material for a third cladding layer and utilizing the
phenomenon shown in FIG. 1 of the Reference 1. However, laser
oscillation was not achieved with this device for comparison. The
reason was that since the third cladding layer was made of GaAs, it
was not possible to obtain any gain necessary for laser
oscillation. This tells that the material for the third cladding
layer 8 must be AlGaAs as explained above.
(Second Embodiment)
FIG. 2 shows a section of a semiconductor laser device representing
a second embodiment of the invention. This semiconductor laser
device includes a p-GaAs substrate 11 and, on (111)B face of the
substrate offset by 2.degree. to a direction of
[100],
an n-GaAs current constrictive layer (1 .mu.m thick) 12, a
p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P first cladding layer
(0.2 .mu.m thick) 13, a non-doped (Al.sub.0.1 Ga.sub.0.9).sub.0.5
In.sub.0.5 P active layer (0.05 .mu.m thick) 14, an n-(Al.sub.0.7
Ga.sub.0.3).sub.0.5 In.sub.0.5 P second cladding layer (1.5 .mu.m
thick) 15, an n-Ga.sub.0.5 In.sub.0.5 P contact layer (0.5 .mu.m
thick) 16, and an n-GaAs contact layer (0.1 .mu.m thick) 17. Shown
by 14a is an oscillation region, and shown by 19 and 20 are
electrodes. A belt-like through-channel 12b extending
perpendicularly to the section is formed centrally in the current
constrictive layer 12, and a p-Al.sub.0.7 Ga.sub.0.3 As third
cladding layer (1.3 .mu.m) 18 is embedded in the through-channel
12b. The surface 18a of the cladding layer 18 is flush with the
surface 12a of the current constrictive layer 12.
This semiconductor laser device is different from the one of the
first embodiment in that the layers 12, 13 . . . are formed on
(111)B face inclined by 2.degree. to the direction of
[100]
and in that the n-Ca contact layer 17 is provided over the n-GaInP
contact layer 16. The provision of the n-GaAs contact layer 17
facilitates ohmic contact with the electrode 20, whereby the
resistance of the device can be reduced. Further, a double
heterostructure consists of the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5
In.sub.0.5 P cladding layer 13, the non-doped (Al.sub.0.1
Ga.sub.0.9).sub.0.5 In.sub.0.5 P active layer 14, the n-(Al.sub.0.7
Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 15. This
arrangement provides an oscillation wavelength of 650 nm.
In fabricating the device, as is the case with the first
embodiment, the substrate temperature was set at 700.degree. C., at
which temperature were formed layers of from cladding layer 18 to
contact layer 17. Despite the fact that layers were formed at an
orientation offset by 2.degree. from (111)B face of the p-GaAs
substrate 11, the surface 18a of the cladding layer 18 could be
made flush with the surface 12a of the current constrictive layer
12. The contact layer 17 was grown on the surface of the contact
layer 16 ((111)B face of GaAs) in such a condition that the
substrate temperature was raised to 740.degree. C. so as to enable
GaAs to grow on (111)B face as well.
This semiconductor laser device, as was the case with the device of
the first embodiment, exhibited good fundamental mode stability.
Current leakage was decreased; oscillation threshold limit was
lowered; and kink level was enhanced. A coating of Al.sub.2
O.sub.3, of .lambda./2 thickness (.lambda. represents oscillation
wavelength) was applied to each end face of a chip of resonator
length of 500 .mu.m. At this condition, the device exhibited
satisfactory characteristics: oscillation threshold value, 50 mA;
and kink level, 45 mW.
(Third Embodiment)
FIG. 3 shows a section of a semiconductor laser device representing
a third embodiment of the invention. The semiconductor laser device
includes a p-GaAs substrate 31 and, on (111)B face of the p-GaAs
substrate 31, an n-GaAs current constrictive layer (1 .mu.m thick)
32, a p-Al.sub.0.7 Ga.sub.0.3 As third cladding layer (0.02 .mu.m
thick) 38', a p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P first
cladding layer (0.2 .mu.m thick) 33, a non-doped Ga.sub.0.38
In.sub.0.62 P active layer (0.02 .mu.m thick) 34, an n-(Al.sub.0.5
Ga.sub.0.5).sub.0.5 In.sub.0.5 P second cladding layer (1.5 .mu.m
thick) 35, and an n-Ga.sub.0.5 In.sub.0.5 P contact layer (0.5
.mu.m thick) 36. Shown by 34a is an oscillation region, and shown
by 39 and 40 are electrodes. A belt-like through-channel 32b
extending perpendicularly to the section is formed centrally in the
current constrictive layer 32, and a p-Al.sub.0.7 Ga.sub.0.3 As
third cladding layer (1.3 .mu.m thick) 38 is embedded in the
through-channel 32b. The cladding layer 38' is an extended portion
which is connected integrally with the cladding layer 38, and coves
the surface of the current constrictive layer 32. The surface 38a
of the cladding layer 38 is flush with the surface 32a of the
current constrictive layer 32.
This semiconductor laser device is different from the device of the
first embodiment in that the composition of the active layer 34 is
non-doped Ga.sub.0.38 In.sub.0.62 P to give some distorted effect,
and in that the substrate temperature was set at 730.degree. C.
during the growth of cladding layer 38 thereby to allow slight
growth of AlGaAs layer 38' on the surface (GaAs (111)B face) 32a of
the current constrictive layer 32 on both sides of the
through-channel 32b. In consequence, the active layer 34 is
slightly bent over the edges of the through-channel 32b as shown.
Thus, the device has a waveguide structure having the
characteristic of an effective refractive index guide utilizing the
light absorption of the substrate (current constrictive layer) and
the characteristic of a real refractive index guide structure
utilizing bending of the active layer, that is, the advantages of
both of the two prior art arrangements show in FIGS. 11 and 12. In
other words, the semiconductor laser device can reduce mode loss
with respect to fundamental mode and enhance mode loss with respect
to higher-order mode, thereby further stabilizing the fundamental
mode. Further, as the device of the first embodiment does, this
device provides for current leakage reduction, oscillation
threshold value decrease, and kink level improvement.
Under the conditions of resonator length 600 .mu.m, and reflection
factors, front side 8% and rear side 70%, the device exhibited an
oscillation threshold value of 55 mA and a kink level of 220 mW
(oscillation wavelength 690 nm). This indicates some twofold
improvement as compared with the kink level (about 120 mW) of the
prior art device shown in FIG. 11 in which same double
heterostructure as the present embodiment is employed.
To fabricate the device, a through-channel 32b is formed in the
current constrictive layer 32 in the same way as in the first
embodiment, and then the substrate temperature is set at
730.degree. C. at which temperature are formed layers including
cladding layer 38 through contact layer 36. With the substrate
temperature so set at 730.degree. C., the cladding layer 38 was
grown within the through-channel 32b in such a manner that its
surface was flush with the surface of the current constrictive
layer 32, so as to fill in the through-groove 32b. Also, an
extension 38' to the cladding layer 38 which was thinner than that
portion of the cladding layer 38 which filled the through-channel
32b was grown on the surface of the current constrictive layer 32.
Further, layers 33, 34, 35 . . . were grown over the cladding layer
38 inclusive of the extension 38' at a moderate rate of growth.
Aforesaid waveguide structure was thus formed. In this case, a
temperature range of 720.degree. to 740.degree. C. is suitable for
the substrate temperature. If the temperature is less than
720.degree. C., it is not possible to grow the extension 38'. If
the temperature is more than 740.degree. C., the extension 38'
grows excessively thick.
(Fourth Embodiment)
FIG. 6 shows a section of a semiconductor laser device representing
a fourth embodiment of the invention. This semiconductor laser
device includes a p-GaAs substrate 41 and, on (111)B face of the
p-GaAs substrate 41, a current constrictive layer 42 (1 .mu.m
thick) consisting of two layers 42', 42", a p-(Al.sub.0.5
Ga.sub.0.5).sub.0.5 In.sub.0.5 P first cladding layer (0.2 .mu.m
thick) 43, a non-doped Ga.sub.0.5 In.sub.0.5 P active layer (0.05
.mu.m thick) 44, an n-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P
second cladding layer (1.5 .mu.m thick) 45, and an n-Ga.sub.0.5
In.sub.0.5 P contact layer (0.5 .mu.m thick). Shown by 44a is an
oscillation region, and shown by 49, 50 are electrodes. A
through-channel 42b extending perpendicularly to the section is
formed centrally in the current constrictive layer 42, and a
p-Al.sub.0.7 Ga.sub.0.3 As third cladding layer (1.3 .mu.m thick)
is filled in this through-channel 42b. The surface 48a of the
cladding layer 48 is flush with the surface 42a of the current
constrictive layer 42.
This semiconductor laser device is different from the device of the
first embodiment in that the current constrictive layer 42 is of a
two-layer construction consisting of n-Al.sub.0.1 Ga.sub.0.9 As
(0.9 .mu.m thick) 42' and n-GaAs (0.1 .mu.m thick) 42". To
fabricate this semiconductor laser device, after n-Al.sub.0.1
Ga.sub.0.9 As (0.9 .mu.m thick) 42' and n-GaAs (0.1 .mu.m thick)
42" are formed, a through-channel 42b is formed centrally in the
current constrictive layer 42 which extends from the surface of the
layer 42" to the substrate 41. Then, the AlGaAs layer 48 is grown
on the substrate 41 by MOCVD process. In the same way as in the
first embodiment, selective growth occurred at a substrate
temperature of less than 720.degree. C. such that only the interior
of the through-channel 42b was filled even when the n-Al.sub.0.1
Ga.sub.0.9 As layer 42' was exposed on the through-channel 42b side
surface of the current constrictive layer 42. Such selective growth
continued until the Al proportion of the AlGaAs current
constrictive layer 42 reached zero or at least 0.3. Subsequently,
layers 43, 44, 45 . . . were grown by MOCVD process. In this way,
it is possible to form an effective refractive index guide
structure utilizing light absorption of the current constrictive
layer 42 and substrate 41, in a well controlled manner.
Just as the devices of the first and second embodiments did, this
semiconductor laser device proved that it was effective for
fundamental mode stabilization, current leak reduction, oscillation
threshold value decrease, and kink level improvement.
(Fifth Embodiment)
FIG. 7 shows a section of a semiconductor laser device representing
a fifth embodiment of the invention. This semiconductor laser
device includes a p-GaAs substrate 51 and, on (111)B face of the
p-GaAs substrate 51, n-GaAs current constrictive layers (1 .mu.m
thick) 52, 52', a p(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P
first cladding layer (0.2 .mu.m thick) 53, a non-doped Ga.sub.0.5
In.sub.0.5 P active layer (0.05 .mu.m thick) 54, an n-(Al.sub.0.5
Ga.sub.0.5).sub.0.5 In.sub.0.5 P second cladding layer (1.5 .mu.m
thick) 55, and an n-Ga.sub.0.5 In.sub.0.5 P contact layer (0.5
.mu.m thick) 56. Shown by 54a is an oscillation region, and shown
by 59, 60 are electrodes. A belt-like through-channel 52b is formed
centrally in the current constrictive layer 52 which extends
perpendicularly to the section. An n-GaAs extension 52' of the
current constrictive layer is embedded in an inner peripheral
portion of the through-channel 52b, and a p-Al.sub.0.7 Ga.sub.0.3
As third cladding layer (1.3 .mu.m thick) is embedded internally of
the current constrictive layer extension 52'. The surface 58a of
the cladding layer 58 is flush with the surfaces 52a, 52a' of the
current constrictive layers 52, 52'.
In fabricating this semiconductor laser device, after a
through-channel (4 .mu.m wide) 52b is formed, an n-GaAs current
constrictive layer extension 52' is laterally grown until the
remaining width of the through-channel 52b is 2.5 .mu.m, and then
an AlGaAs fill layer 58 is grown. Subsequently, in the same way as
in the first embodiment, layers 53, 54, 55 . . . are grown.
This semiconductor laser device, just as the device of the first
embodiment does, has an effective refractive index guide structure
formed in a well controlled manner, and provides for good
fundamental mode stability, current leak reduction, oscillation
threshold value decrease, and kink level enhancement. Furthermore,
the provision of the current constrictive layer extension 52'
formed in the through-channel 52b' so as to reduce the width of
current injection path results in greater reduction of oscillation
threshold value than in the case of the device of the first
embodiment, and further in reduced astigmatism. Whereas, under
conditions of non-coat, resonator length of 400 .mu.m, the device
of the first embodiment exhibited an oscillation threshold value of
40 mA and an astigmatism of 5 .mu.m (3 mWh), the semiconductor
laser device of the present embodiment exhibited an oscillation
threshold value of 30 mA and an astigmatism value of 0 .mu.m (3
mWh) under the same conditions.
(Sixth Embodiment)
FIG. 8 shows a section of a sixth embodiment of the invention. This
semiconductor device includes a p-GaAs substrate 61 and, on (111)B
face of the p-GaAs substrate 61, an n-GaAs current constrictive
layer (1 .mu.m thick) 62, a p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5
In.sub.0.5 P first cladding layer (0.2 .mu.m thick) 63, a non-doped
Ga.sub.0.5 In.sub.0.5 P active layer (0.05 .mu.m thick) 64, an
n-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P second cladding
layer (1.5 .mu.m thick) 65, and an n-Ga.sub.0.5 In.sub.0.5 P
contact layer (0.5 .mu.m thick) 66. Shown by 64a is an oscillation
region, and shown by 69, 70 are electrodes. Three belt-like
through-channels 62b, 62b, 62b (3.5 .mu.m wide each, arranged at
5.5 .mu.m pitch) which extend perpendicularly to the section are
respectively formed centrally and on both sides of the current
constrictive layer 62. A p-Al.sub.0.7 Ga.sub.0.3 As third cladding
layer (1.3 .mu.m thick) 68 is filled in each of the
through-channels 62b. The surface 68a of the cladding layer 68 is
flush with the surface 62a of the current constrictive layer
62.
This semiconductor laser device is identical with the device of the
first embodiment insofar as layer arrangement is concerned, but is
different from the latter in that it includes a plurality of
through-channels 62b formed in the current constrictive layer 62
which serve as current path, and accordingly it constitutes a
semiconductor laser array having a plurality of oscillation regions
64a. With this semiconductor laser device, oscillation at
180.degree. phase mode was observed up to such a high output as 350
mW.
To fabricate this semiconductor device, after current constrictive
layer 62 is formed on the substrate 61, a plurality of
through-channels 62b, 62b, 62b are formed simultaneously in the
current constrictive layer 62. Thereafter, in the same way as in
the first embodiment, third cladding layers 68 are filled in the
respective through-channels 62b. Then, layers 63, 64, 65 . . .
which make a double heterostructure are grown.
(Seventh Embodiment)
FIG. 9 shows a section of a semiconductor laser device representing
a seventh embodiment of the invention. This semiconductor laser
device includes a p-GaAs substrate 71 and, on (111)B face of the
p-GaAs substrate 71, an n-GaAs current constrictive layer (1 .mu.m
thick) 72, a p-(Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P first
cladding layer (0.2 .mu.m thick) 73, a non-doped Ga.sub.0.5
In.sub.0.5 P active layer (0.05 .mu.m thick) 74, an n-(Al.sub.0.5
Ga.sub.0.5).sub.0.5 In.sub.0.5 P second cladding layer (1.5 .mu.m
thick) 75, and an n-Ga.sub.0.5 In.sub.0.5 P contact layer (0.5
.mu.m thick) 76. Shown by 74a is an oscillation region, and shown
by 79 and 80 are electrodes. A belt-like through-channel 72b
extending perpendicularly to the section is formed centrally in the
current constrictive layer 72, and a p-Al.sub.0.7 Ga.sub.0.3 As
third cladding layer (1.3 .mu.m) 78 is filled in the
through-channel 72b (which serves as a current path). Belt-like
non-through channels 72' are formed in plurality at a predetermined
pitch at both sides of the current constrictive layer 72. Filled in
respective non-through channels 72b' are p-Al.sub.0.7 Ga.sub.0.3 As
fourth cladding layers. The surface of each cladding layer 78, 78'
is flush with the surface 72a of the current constrictive layer
72.
To manufacture the semiconductor laser device, the current
constrictive layer 72 is first formed on the substrate 71, and then
non-through channels 72b', . . . which are to be retained within
the current constrictive layer 72 are formed in the current
constrictive layer 72. Then, a through-channel 72b which extends
from the surface of the current constrictive layer 72 to the
substrate 71 is formed in the current constrictive layer 72.
Subsequently, in the same way as in the first embodiment, the
substrate 1 is kept at a temperature of 700.degree. C. In this
condition, cladding layers 78, 78', . . . are grown simultaneously
within the through-channel 72b and non-through channels 72b' in
such a manner that their respective surfaces are flush with the
surface of the current constrictive layer 72, to thereby fill the
through-channel 72b and non-through channels 72b'. After that, in
the same way as in the first embodiment, cladding layer 73, active
layer 74, and cladding layer 75 are grown all over.
This semiconductor laser device is identical with the device of the
first embodiment in layer construction, except that through-channel
72b and non-through channels 72b' . . . are formed in plurality in
the current constrictive layer 72. The centrally located
through-channel 72b extends from the surface of the current
constrictive layer 72 to the substrate 71, while the non-through
channels 72b' are retained within the current constrictive layer
72. Through-channel 72b is the only through-channel for defining an
oscillation region, while other channels, that is, non-through
channels 72b', . . . are intended to eliminate possible distortion
arising from the layer structure. In the semiconductor laser device
of the first embodiment which is shown in FIG. 1, all strain is
applied to that portion of the active layer 74 which is located
above the edges of the through-channel 2b (boundary of the
oscillation region 4a). In the present embodiment, the presence of
non-through channels 72b' (and AlGaAs layers filled in these
channels), in addition to the through-channel 72b, provides for the
dispersion of the strain exerted upon the active layer 74 into the
non-through channels 72b', thus alleviating the strain of the
oscillation region 74a, which fact insures good long-term
reliability of the device.
(Eighth Embodiment)
FIGS. 10A and 10B show a surface-output type light emitting diode
representing eighth embodiment of the invention, FIG. 10A being a
sectional view, FIG. 10B being a top plan view with respect to FIG.
10A section. This light-emitting diode includes a p-GaAs substrate
81 and, on (111)B face of the p-GaAs substrate 81, an n-GaAs
current constrictive layer 82, a p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5
In.sub.0.5 P first cladding layer 83, a non-doped (Al.sub.0.45
Ga.sub.0.55).sub.0.5 In.sub.0.5 P active layer 84, an n-(Al.sub.0.7
Ga.sub.0.3).sub.0.5 In.sub.0.5 P second cladding layer 85, and an
n-Ga.sub.0.5 In.sub.0.5 P contact layer 86. Shown by 89, 90 are
electrodes. A through-channel 82b of a circular pattern is formed
centrally in the current constrictive layer 82, and in this
through-channel 82b is filled a p-Al.sub.0.7 Ga.sub.0.3 As third
cladding layer 88. The surface 88a of the cladding layer 88 is
flush with the surface 82a of the current constrictive layer 82. A
center portion of the cladding layer 85 is worked to a
frusto-conical pattern by the ion milling technique.
The current constrictive layer 82, through-channel 82b, and
cladding layer 88 are formed in substantially the same way as in
the first embodiment. Therefore, the cladding layer 83, active
layer 84, and cladding layer 85 which constitute a double
heterostructure are formed in a well controlled manner, and this
provides for good stability in light emission intensity-applied
current characteristics. No etching step is involved after the
formation of the cladding layer 88 and, therefore, it is unlikely
that any growth interface will become oxidized. Thus, possible
current leakage is reduced and light emission intensity is
increased. In this way, improvement can be effected with respect to
light-emitting diode characteristics. A contact layer 86 and an
upper electrode 90 are defined by a small circular pattern disposed
on a frusto-conical portion of the cladding layer 85. This provides
for increased efficiency of drawing light from the device
surface.
A molded package of 5 mm dia. incorporating the light-emitting
diode, when energized 20 mA, exhibited a luminous intensity of 4
candela (i. e., conventionally of the order of 0.3 candela) at a
wavelength of 555 nm.
In the foregoing embodiments, the GaAs substrate is of the p-type,
but it is needless to say that the substrate is not so limited. The
GaAs substrate may be of either p-conductivity type or
n-conductivity type, and the conductivity type of each respective
layer grown may be determined depending upon the conductivity type
of the GaAs substrate. The wave band of laser oscillation may be
selected from the wave band range of red to orange, by suitably
selecting the composition of the AlGaInP active layer. The active
layer need not necessarily be non-doped, but may be of the p-type
or of the n-type. For the double hetero layer structure, where so
required, may be employed an SCH structure (separate confinement
heterostructure) including a guide layer as required, or a
multiquantum well structure or multiquantum barrier structure. A
GaInP or GaAs contact layer may be provided as required. The GaAs
substrate need not necessarily be oriented in a just direction
insofar as it has (111)B face as a main face, but may be oriented
some degrees off in
[100] or [011] directions.
For layer growth after the AlGaAs layer is filled in the current
constrictive layer, MOCVD process is preferably employed, but other
vapor phase growing methods may be employed instead, including
molecular beam epitaxy, atomic layer epitaxy, and chemical beam
epitaxy methods.
As is clear from the foregoing description, a semiconductor
light-emitting device of the invention includes a substrate having
one of p- and n-conductivity types, a current constrictive layer
formed on the surface of the substrate and having the other of the
p- and n-conductivity types, the current constrictive layer having
a through-channel extending to the surface of the substrate in a
direction perpendicular to the surface of the substrate for
defining a current path extending perpendicularly to the surface of
the substrate, and a double heterostructure formed on the current
constrictive layer including a first cladding layer, an active
layer and a second cladding layer, wherein the through-channel is
of a belt-like pattern which extends perpendicularly to the surface
of the substrate. Therefore, according to this arrangement, it is
possible to provide a refractive index guide type semiconductor
laser device. The semiconductor laser device has a third cladding
layer of the one conductivity type filled in the through-channel of
the current constrictive layer, the surface of the third cladding
layer being flush with the surface of the current constrictive
layer. The first cladding layer, active layer and second cladding
layer which constitute the double heterostructure are formed by a
known growth method, for example, MOCVD process, flatly over the
current constrictive and third cladding layers in a well-controlled
manner, without involving any process of etching. Therefore, the
first cladding layer, active layer and second cladding layer
involve almost no variation in thickness. This insures better
stability in the fundamental mode of laser oscillation, as compared
with any conventional semiconductor laser device. The fact that no
etching process is involved after the formation of the third
cladding layer eliminates the possibility of any growth interface
being oxidized. This results in decreased current leakage and
lowered oscillation threshold limit. The first cladding layer is
usually designed to be relatively thin and in no case does this
result in decreased higher-order mode loss. Therefore, good
improvement is achieved in kink level. In this way, a semiconductor
laser device having improved performance characteristics can be
obtained.
Where the third cladding layer has an extended portion covering the
surface of the current constrictive layer, the extended portion
having a thickness set thinner than the portion filled in the
through-channel, the active layer is so formed that it is slightly
bent adjacent the edge of the through-channel, when the double
heterostructure is formed as by the MOCVD process. As a result, a
waveguide arrangement is obtained which has both the characteristic
of an effective refractive index guide structure utilizing the
light absorption of the substrate (current constrictive layer) and
the characteristic of a real refractive index guide structure
utilizing the bend of the active layer. This provided for mode loss
reduction with respect to the fundamental mode and mode loss
enhancement with respect to the higher-order mode, with the result
that the fundamental mode is further stabilized.
Where an extension to the current constrictive layer having the
other type of conductivity is filled in a peripheral portion of the
through-channel, the surface of the extension being flush with the
surface of the current constrictive layer, and the third cladding
layer is filled internally of the extension to the current
constrictive layer within the through-channel, during operation of
the device, the width of current injection can be proportionally
reduced by the extension to the current constrictive layer.
Therefore, the oscillation threshold value is further reduced and,
in addition, some astigmatism reduction is achieved.
Where a plurality of through-channels are formed in the current
constrictive layer, with the third cladding layers being embedded
in the respective through-channels, during operation of the device,
a plurality of oscillation regions develop according to the number
of current paths formed by the through-channels. This provides a
semiconductor laser array.
Where at least one non-through channel is formed in parallel with
the through-channel in the current constrictive layer to a depth
not greater than the depth of the current constrictive layer, and a
fourth cladding layer having the one type of conductivity is filled
in the non-through channel, the surface of the fourth cladding
layer being flush with the surface of the current constrictive
layer, any strain which is exerted upon the active layer because of
layers being stacked on the substrate can be distributed over the
non-through channel. Thus, the strain applied to the oscillation
region over the through-channel can be alleviated so that good
improvement is achieved in the long-term reliability of the
device.
The semiconductor light-emitting device of an embodiment of the
invention includes a substrate having one of p- and n-conductivity
types, a current constrictive layer formed on the surface of the
substrate and having the other of the p- and n-conductivity types,
the current constrictive layer having a through-channel extending
to the surface of the substrate in a direction perpendicular to the
surface of the substrate for defining a current path extending
perpendicularly to the surface of the substrate, and a double
heterostructure formed on the current constrictive layer including
a first cladding layer, an active layer and a second cladding
layer, wherein the through-channel is of a circular pattern.
According to this arrangement, it is possible to provide a surface
output type light-emitting diode. The light-emitting diode
comprises a third cladding layer having the one type of
conductivity and filled in the through-channel of the current
constrictive layer, the surface of the third cladding layer being
flush with the surface of the current constrictive layer.
Therefore, the first cladding layer, active layer and second
cladding layer which constitute the double heterostructure are
formed by a known growth method, for example, MOCVD process, flatly
over the current constrictive and third cladding layers in a
well-controlled manner, without involving any process of etching.
Therefore, the first cladding layer, active layer and second
cladding layer involve almost no variation in thickness. This
insures good stability in radiation intensity-applied current
characteristics. The fact that no etching process is involved after
the formation of the third cladding layer eliminates the
possibility of any growth interface being oxidized. This results in
decreased current leakage and increased light emission intensity.
In this way, a light-emitting diode having improved performance
characteristics can be obtained.
Where the layers forming the double heterostructure are formed to
be frusto-conical, the efficiency of light output of the device can
be enhanced.
Also, the substrate is a GaAs substrate having (111)B face or a
face offset to the (111)B face which is a main face; the current
constrictive layer is comprised of GaAs or AlGaAs; and the third
cladding layer is comprised of AlGaAs. In this case, it is possible
to grow the third cladding layer comprised of AlGaAs having the one
type of conductivity within the through-channel while the substrate
is kept at a temperature of not more than 720.degree. C. to fill
the through-channel with the third cladding layer in such a manner
that the surface of the third cladding layer becomes flush with the
surface of the current constrictive layer. Therefore, the first
cladding layer, active layer, and second cladding layer which
constitute a double heterostructure can be grown by the known
growth technique flatly over the current constructive layer and
third cladding layer in a well controlled manner.
According to a method of making a semiconductor light-emitting
device of an embodiment of the present invention, on a surface of a
GaAs substrate having one of p- and n-conductivity types is formed
a current constrictive layer comprised of GaAs or AlGaAs and having
the other of the p- and n-conductivity types, the substrate surface
being (111)B face or a face offset to the (111)B face which is a
main face. A through-channel of a predetermined pattern which
extends from the surface of the current constrictive layer to the
substrate is then formed in the current constrictive layer. Then, a
third cladding layer comprised of AlGaAs and having the one type of
conductivity is grown within the through-channel while the
substrate is kept at a temperature of not more than 720.degree. C.
This enables the third cladding layer to be grown so that its
surface becomes flush with the surface of the current constrictive
layer thereby to fill the through-channel. Therefore, the first
cladding layer, active layer, and second cladding layer can be
formed flatly over the third cladding layer and current
constrictive layer in a well controlled manner to form a double
heterostructure. Thus, it is now possible to fabricate
semiconductor light-emitting devices, such as semiconductor laser
devises and light-emitting diodes, which have improved performance
characteristics as compared with conventional devices of the
kind.
According to the method of making a semiconductor light-emitting
device of an embodiment, after a through-channel is formed in a
current constrictive layer, a third cladding layer comprised of
AlGaAs and having one type of conductivity is grown while a
substrate is kept within a temperature range of 720.degree. C. to
740.degree. C. so as to fill the through-channel in such a manner
that a surface of portion of the third cladding layer which fills
the through-channel is flush with a surface of the current
constrictive layer and that the third cladding layer has an
extended portion overlying the surface of the current constrictive
layer and being thinner than the fill portion. Therefore, when
layers constituting a double heterostructure are formed on the
current constrictive layer using, for example, the MOCVD technique,
an active layer is formed to be slightly bent adjacent an edge of
the through-channel. Thus, the semiconductor light-emitting device
made has a waveguide construction featuring both the characteristic
of an effective refractive index guide structure utilizing the
light absorption of the substrate (current constrictive layer) and
the characteristic of a real refractive index guide structure
utilizing the bend of the active layer. Therefore, the
semiconductor laser device can reduce mode loss relative to the
fundamental mode and increase mode loss relative to the
higher-order mode, so that the fundamental mode is further
stabilized. Furthermore, possible current leaks are reduced and the
oscillation threshold limit is lowered.
According to the method of making a semiconductor light-emitting
device of an embodiment, an extension to a current constrictive
layer which has the other type of conductivity and whose surface is
flush with a surface of the current constrictive layer is filled
inside a through-channel in the current constrictive layer, and the
third cladding layer is filled internally of the extension to the
current constrictive layer. Therefore, when the semiconductor
light-emitting device thus made is in operation, a width of current
injection is proportionally reduced by the extension to the current
constrictive layer. With such semiconductor light-emitting device,
and semiconductor laser device in particular, therefore, the
oscillation threshold value is further reduced and, in addition,
some astigmatism reduction is achieved.
According to the method of making a semiconductor light-emitting
device of an embodiment, through-channels are formed in plurality
in a current constrictive layer, and third cladding layers are
filled in respective through-channels. In operation of such a
semiconductor light-emitting device, a plurality of oscillation
regions develop according to the number of current paths formed by
the through-channels. This provides a semiconductor laser
array.
According to the method of making a semiconductor light-emitting
device of an embodiment, at least one non-through channel having a
depth of not more than a depth of a current constrictive layer is
provided in parallel with a through-channel in the current
constrictive layer, and a fourth cladding layer having the one type
of conductivity is embedded in the non-through channel, with a
surface of the fourth cladding layer being made flush with a
surface of the current constrictive layer. By virtue of this
arrangement, any strain exerted on an oscillation region over the
through-channel is alleviated so that good improvement can be
obtained in the long-term reliability of the device.
According to the method of making a semiconductor light-emitting
device of an embodiment, after a through-channel of a circular
pattern which extends from a surface of a current constrictive
layer to a substrate is formed in the current constrictive layer, a
third cladding layer having one type of conductivity is grown
within the through-channel while the substrate is kept at a
temperature of not more than 720.degree. C., in such a manner that
a surface of the third cladding layer is flush with the surface of
the current constrictive layer, thereby filling the
through-channel. Further, a first cladding layer, an active layer,
and a second cladding layer are grown over the current constrictive
layer and third cladding layer to form a double heterostructure.
Thus, a surface output type light-emitting diode is constructed. In
this case, the layers which constitute the double heterostructure
are flatly formed by a known growth method, for example, MOCVD
process, in a well-controlled manner, without involving any process
of etching. Therefore, almost no variation is involved in
thickness. This insures good stability in radiation
intensity-applied current characteristics. Furthermore, since no
etching process is involved after the formation of the third
cladding layer, there is no possibility of any growth interface
being oxidized. This results in decreased current leakage and
increased light emission intensity. In this way, a light-emitting
diode having improved performance characteristics can be obtained.
Moreover, because of the fact that the layers forming the double
heterostructure are formed to be frustoconical, the efficiency of
light output of the device can be enhanced.
The invention being thus described, it will be obvious that the
same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are intended to be included within the scope of the
following claims.
* * * * *